1. Field of the Invention
The present invention generally relates to a proximity IC card (hereunder sometimes abbreviated to xe2x80x9cPICCxe2x80x9d) and, more particularly, to a circuit for detecting the presence or absence of a PICC in a PICC reader/writer (hereunder abbreviated as PICC-R/W) for writing data to and reading data from the PICC.
2. Description of the Related Art
PICC standards are described in ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 14443. Hereinafter, in relation to the present invention, a brief description is given about part of ISO/IEC 14443, which relates to a type-B communication interface for a PICC and which describes the properties and characteristics of a field that provides power transmission and bidirectional communication between a PICC and a proximity coupling device (hereunder abbreviated to PCD), such as the PICC-R/W.
(1) Power Transmission from PCD to PICC
To supply effective power to the PICC in a radio frequency (RF) operating field, a carrier (having a carrier frequency of fc=13.56 MHz)is transmitted from the PCD to the PICC, whereupon the received carrier is rectified to thereby generate electric power needed for an operation of an internal circuit.
(2) Communication from PCD to PICC
The PCD transmits data to the PICC by amplitude-shift-keying (ASK) modulating the amplitude of the carrier with a modulation index of 10% at a data bit rate of 106 Kbps (=fc/128).
(3) Communication from PICC to PCD
The PICC transmits data to the PCD by performing load modulation of a load for reception of the carrier at a frequency fa (=fc/16), which is (1/16) the carrier frequency, to thereby generate a subcarrier (whose frequency fs=847 kHz), and by then binary-phase-shift-keying (BPSK) modulating the phase of the subcarrier at a data bit rate of 106 Kbps (=fc/128).
FIG. 1 schematically shows the constitution of an example of a PICC.
In the case of the example of FIG. 1, two chips respectively constituting a central processing unit (CPU) portion 11 and an RF portion 12 are incorporated into a card body 10. Further, an antenna (AT) 13, wound like a coils is disposed along the periphery of the card body 10. The CPU portion 11 is constituted by what is called a one-chip computer and includes a CPU, memories such as a ROM, a RAM, and an EEPROM, and an input/output (I/O) interface.
FIG. 2 shows the constitution of an example of the communication interface between a PCD and a PICC.
In the case of the communication from the PCD to the PICC, which has been described in the foregoing section (2), a modulation portion (MOD) 20 of the PCD performs ASK modulation of the amplitude of a carrier (having the carrier frequency of fc=13.56 MHz) with a modulation index of 10%. Then, a resultant signal is transmitted from the PCD to the PICC through output amplifiers 22 and 23 and an antenna 24.
In contrast, in the case of the communication from the PICC to the PCD, which has been described in the foregoing section (3), a load 27 for reception of an RF signal is varied under the control of a modulation portion (MOD) 29 of the RF portion 12 of the PICC shown in FIG. 1. Then, a BPSK modulation for providing binary phase information (representing 0 or 180 degrees) is performed on a subcarrier (whose frequency fs=847 kHz) generated by a load modulation (resulting in an amplitude modulation (hereunder referred to as an AM modulation)).
The modulated signal is transmitted to the PCD through an antenna 26 (corresponding to the antenna 13 of FIG. 1). Actually, a detection portion (DET) 21 of the PCD detects the carrier that is outputted by the PCD itself and that undergoes the load modulation (including the BPSK modulation) performed by the PICC, as illustrated in FIG. 2.
A photosensor portion 25 of the PCD is used for verifying the presence of the PICC within a communication range of the PCD. For example, the photosensor portion is provided in a card detector of the PCD, and adapted to detect a PICC, which passes through a card insertion portion or which is placed in a card slot, by using optical components, such as a photodiode.
As described above, in the conventional communication interface circuit, means for verifying the presence of a PICC within the communication range of a PCD is provided as a circuit using an optical component other than a data transmitting/receiving circuit. Thus, the conventional communication interface circuit has drawbacks in that the cost of components thereof increases and that the component mounting space thereof is limited. However, in recent years, PICCs have come into wide use, so that PICCs now cover a wide field of application. It is eagerly requested that the size and cost of not only PICCs but PICC-R/W are decreased as much as possible. Thus, first, it is necessary to reduce the number of components as much as possible.
Furthermore, if an iron plate is used instead of a PICC, conventional PICC detecting means comprising the photosensor portion 25 determines that a PICC is present.
In this case, spatial impedance within the RF field becomes extraordinarily low owing to the presence of an electric conductor or a magnetic substance, such as the iron plate. Consequently, the conventional communication interface circuit has a drawback in that the PICC-R/W outputs excessive carriers, and thus electronic components are damaged after the output of the carriers.
Thus, the conventional communication interface circuit has an additional circuit 25xe2x80x2 that is used for discriminating the card and that is dedicated to discrimination of a PICC from other electric conductors and magnetic substances. This results in a rise of the aforementioned cost of components, and in an increase in the limitation on the mounting space.
Accordingly, in view of the aforementioned drawbacks of the conventional detection circuit, an object of the present invention is to provide a circuit for detecting the presence or absence of a PICC, which simultaneously performs both the detection of the presence or absence of a PICC and the discrimination of the PICC by using a simple circuit structure formed in such a manner as to be integral with the existing detection part (DET) 21 of a data transmitting/receiving portion, different from the conventional communication interface circuit in which the circuit 25 dedicated to the detection of the presence or absence of a PICC is formed in such a way as to be separated from the circuit 25xe2x80x2 dedicated to the discrimination of a PICC.
To achieve the foregoing object, according to the present invention, there is provided a circuit for detecting the presence or absence of a proximity IC card. This circuit comprises an antenna for receiving a carrier signal outputted to a proximity IC card, D.C. detecting means for rectifying a carrier signal received from the antenna and for detecting a D.C. component of the received carrier signal, and level judgment means for determining the level of the D.C. component detected by the D.C. detecting means and for judging, when the level of the D.C. component is within a predetermined range, that a proximity IC card is present in an RF field.
Further, the D.C. detecting means is a detection circuit for detecting a subcarrier signal that is sent from a proximity IC card and superposed onto the carrier signal. The detection circuit outputs the D.C. component and the subcarrier signal superposed thereonto as rectification outputs obtained by rectification of the carrier signal.
Furthermore, the detection circuit comprises a bias circuit for applying predetermined D.C. potential to a signal received from the antenna, a rectifier circuit for extracting a subcarrier signal superposed onto the carrier signal by rectifying a signal received from the antenna at the D.C. bias point, and an amplifier circuit for amplifying the subcarrier signal extracted at the D.C. bias point.
Moreover, the level judgment means is operative to judge, when the level of the D.C. component sent from the detection circuit is not less than a first level, that a proximity IC card is not in the RF field. Furthermore, the level judgment means is operative to judge, when the level of the D.C. component sent from the detection circuit is not more than the first level and not less than a second level, that a proximity IC card is present in the RF field. Further, the level judgment means is operative to judge, when the level of the D.C. component sent from the detection circuit is not more than the second level, that an object other than the proximity IC card is present in the RF field.
Additionally, the level judgment means has thresholds of a Schmitt trigger type for preventing variation in result of judgment on the level in the case when the level of the D.C. component is in the vicinity of the first or second level. Alternatively, the level judgment means has a judgment mask timer circuit, whose masking time is not less than the period of the subcarrier signal, for preventing a variation in a result of judgment on the level in such a case.
The circuit for detecting presence or absence of a proximity IC card is provided in the proximity coupling device.